1. Field of the Invention
The present invention relates to an MgB2 compound sheath superconducting wire capable of producing a high superconducting critical current density by using a superconductor causing superconductivity in an environment below the critical temperature and its manufacturing method.
More specifically, the present invention is applied to a current lead, a transmission line, a large-size magnet, a nuclear magnetic resonance analysis device, a medical MRI device, a superconducting power storage device, a magnetic separation device, a device for pulling crystal up in magnetic field, a superconducting magnet device used for cooling in refrigerator, a superconducting energy storage, a superconducting generator, a fusion reaction furnace magnet, and a magnetically levitated train.
2. Description of the Related Art
Conventionally, as a superconducting material, metal-based materials such as niobium titanium (NbTi) and niobium 3 tin (Nb3Sn) are known. However, among these metal-based superconducting materials, even the niobium 3 germanium (Nb3Ge) having the highest critical temperature is 23K (Kelvin) and there has been problem that it is necessary to use expensive liquid helium for cooling.
As for the high temperature superconductor, a lanthanum (La) oxide-based superconductor having the critical temperature of 30K was discovered in April, 1986. Since then, various copper oxide-based superconductors contain yttrium (Y)-based, bismuth (Bi)-based, thallium (Tl)-based, mercury (Hg)-based bodies having the critical temperature exceeding the boiling point of the liquid nitrogen (77K) have been discovered.
However, since these copper oxide-based superconductors are composed of so-called ceramics, they are therefore low workability to produce a lengthy and uniformly excellent line material, which is problem.
Moreover, since the copper oxide-based superconductor has a large magnetic field anisotropy, it is necessary to align the crystal orientation between the substrate or the sheath material and the superconducting layer. That is, it is necessary to use those having a small mismatch between the crystal orientation between the substrate or sheath material and the superconductor, and also use those orientated three axes of the crystal orientations. This is why the control for such requirements describe above is very difficult and there are a lot of problems for yield and cost.
In the 21st century, as is reported in Nature 410, 63-64 (2001), it was discovered that magnesium diboride (MgB2) causes superconductivity at about 40K. Since this material has quite small magnetic field anisotropy, it is possible to obtain a high critical current density without aligning the crystal orientation with the substrate. Moreover, the material has an excellent bending characteristic. Even when 1.5% of distortion is applied to the superconducting wire, it is possible to maintain the critical current density of 90% with respect to the wire without distortion. This has been proved by an experiment made by the inventors of the present application.
Moreover, there is an advantage that the critical temperature is higher than that of the metal-based superconducting material by more than 20K. Cond-mat/0108265 reports that the upper critical magnetic field is about 40 T in a thin film. If these characteristics are utilized, application in a strong magnetic field is considered to be realistic.
Furthermore, it has been appreciated that this material has a significantly effective characteristic for practice that it is possible to obtain a practically critical current density only by mechanical processing. That is, it is possible to obtain a high superconductivity without performing thermal treatment in the manufacturing process. This characteristic is completely different from that of the conventional superconducting wire which does not cause a superconducting phenomenon without applying thermal treatment. This characteristic can be used for realizing the following:                (1) Reducing the manufacturing steps        (2) Increasing the selection range of the metal sheath material        (3) Improving degree of design freedom of the coil windingAccordingly, it is considered that it is possible to significantly reduce the cost as compared to the conventional superconducting material.        
Moreover, when manufacturing a metal-based superconducting wire and an oxide superconducting wire, it is possible to manufacture a wire by the extruding and drawing used in general. Accordingly, this material is a quite attractive.
If a practical superconducting characteristic with an MgB2-based wire can be maintained, it can be applied not only to a power transmission cable but to various fields including a bio-science spot lighted recently.
However, the MgB2-based superconducting wire has a problem that the critical current density is low as compared with the conventional metal-based and the oxide-based superconducting wire. Moreover, a multi-core wire has also been examined but not yet reached at the practical critical current density.
In order to solve the aforementioned problems, various studies have been conducted: the MgB2-based material is synthesized under a high pressure, the MgB2-based material is made into a single phase, metal powder is added to the MgB2-based material so as to improve junction between crystal grains, a pinning center is introduced, and the like. As a result, a short wire can cause a critical current density of the practical level. However, in the case where the actual application of the superconducting magnet is considered, it is necessary to obtain a high stability and high strength in addition to the critical current density. However, at the current stage, no MgB2-based material has solved these technical problems.